Method for access to a medium by a multi-channel device

ABSTRACT

Method of accessing a medium on a transmission system having at least two channels, in which a multi-channel device groups at least two channels for the purpose of increasing the bandwidth. The method enables multi-channel devices and single-channel devices to coexist by sending out a preamble and header on each individual channel before the channels are grouped, a single-channel device that may be scanning the channel thus detecting that the message that follows is not intended for itself.

The invention relates to methods for access to a medium by amulti-channel device. The medium comprises a transmission system havingat least two channels on which a message to be transmitted comprises atleast a preamble and a header plus a succeeding data field. The datafield may contain either user data, which is termed “useful data”, orcontrol information, for coordinating the access to the medium forexample. A method for decentralized medium access and a method forcentralized medium access are claimed.

The frequency band of a transmission system is often divided intosub-bands, which are termed channels, on which either a singlecommunications link or else a complete cell of a system operates. Thelatter is the case with for example 802.11a/e and HiperLAN/2 wirelesslocal area networks (WLANs). With regard to 802.11a/e WLANs, the basicspecification, namely ANSI/IEEE Std 802.11, 1999 edition, and “IEEE Std801.11a-1999: High-speed Physical Layer in the 5 GHz Band, Supplement toStandard IEEE 802.11”, IEEE New York, September 1999 and “IEEE Std802.11e/D4.2: Medium Access Control (MAC) Enhancements for Quality ofService (QoS), Draft Supplement to Standard IEEE 802.11”, IEEE New York,February 2003, are hereby incorporated by reference.

The bandwidth of the channel sets a limit to the maximum data rate thatcan be obtained between two stations in a cell, or in other words to thecapacity of a cell. One possible way of increasing the capacity of atransmission system is to enlarge the bandwidth of a communicationschannel. With a preset channel definition, this can be achieved bygrouping two or more channels to obtain one channel of greater width.This approach is known in theory and has been implemented in certainWLAN systems, in which a high data rate is obtained or a what is called“turbo” mode of handling.

There are a certain number of stations in a transmission system, and ofthese two or more at a time make a temporary connection with oneanother. The grouping of channels can only be performed if all thestations involved operate in the mode where the data rate is high. Forchannel grouping, there has to be one standard for all the stations orterminals. However, mobile telecommunications is the very area in whichthere are a variety of manufacturers of terminals and the terminalsemploy channel grouping that is not standardized. Because channelgrouping, as a performance feature, entails additional expenditure indevelopment and manufacture, it may also be the case that, because ofthe cost, a manufacturer of terminals offers on the one hand terminalsemploying channel grouping and on the other hand terminals that do nothave this performance feature. Also, older terminals that were developedand sold before channel grouping was introduced in transmission systemsare incapable of operating in the mode having the high data transmissionrate.

Standard IEEE 802.11e combines both a decentralized and a centralizedscheme for medium access. An algorithm that can be used for this will beexplained by taking the 802.11 system as an example. The basic 802.11MAC protocol is the distributed coordination function (DCF), thatoperates as a listen-before-talk scheme. The distributed coordinationfunction is based on carrier multiple sense access (CMSA). Having foundthat no other transmissions are underway on the wireless medium, thestations emit MAC service data units (MSDUs) of an arbitrary length ofup to 2304 bytes. If, however, two stations find that a channel is freeat the same time, a collision occurs when the data is sent out over theradio-transmitting medium, namely air, that they both use. The 802.11MAC protocol defines a mechanism for collision avoidance (CA) to reducethe probability of collisions of this kind. It is part of the collisionavoidance mechanism that, before it starts transmitting, a stationperforms a waiting or backoff process. The station continues listeningout on the channel for an additional, random period of time after itfinds that the channel is free. Only if the channel remains free forthis additional random length of time is the station permitted toinitiate a transmission. This random waiting time is composed of aconstant portion, the what is called DCF interframe space (DIFS), whichis 34 μs long in the case of the 802.11a MAC protocol, and a randomportion whose length is between zero and a maximum time. The DIFS spaceis thus the minimum possible waiting time for the stations. The lengthof the random portion of the waiting time is obtained as a multiple ofthe length of a time slot (slot time), which length is 9 μs in the802.11a MAC protocol. Each station draws a random value, for the numberof slot times to be waited, which it stores in the what is calledcontention window (CW). On the expiry of each period of 9 μs, the valueof the CW is decremented by 1.

Each time a data frame is successfully received, the receiving stationimmediately sends out an acknowledgement frame (ACK). The size of thecontention window is enlarged if a transmission fails, which means thata data frame sent out has not been acknowledged. After each failedattempt at a transmission, a new medium access is effected after a freshwaiting time, with the fresh waiting time being selected to be twice aslong as the current contention window. This reduces the likelihood of acollision in the event that a plurality of stations are trying to gainaccess to the channel. Those stations that deferred channel accessduring the time when the channel was busy do not select a new randomwaiting time but continue the countdown of the time for the deferredmedium access on finding that the channel is idle again. In this way,stations that deferred channel access due to their longer random waitingtime as compared with other stations are given a higher priority whenthey resume their efforts to start a transmission. After each successfultransmission, the transmitting station performs a new random waitingprocess (backoff) even if it does not have a further MSDU to send at thetime. This is what is called the “post backoff”, because this waitingprocess takes place after rather than before a transmission.

There is a situation under the 802.11 MAC protocol in which a stationdoes not have to perform a waiting process of random duration (abackoff) before it can start transmitting data. This situation arises ifan MSDU from a higher layer arrives at a station and the post-backofffor the last transmission has already been completed, or in other words,there is no queue and, in addition, the channel has been idle for aminimum DIFS time. All subsequent MSDUs that arrive after this MSDU willbe transmitted after a random waiting time until there is again noqueue.

To limit the probability of long frames colliding and being transmittedmore than once, data frames are also fragmented. A long MSDU can bedivided by fragmentation into a plurality of small data frames, i.e.fragments, that can be transmitted sequentially as data frames to beacknowledged individually. The advantage of fragmentation is that, if atransmission fails, this failure can be detected at an earlier point intime and hence less data has to be re-transmitted.

In systems using CSMA, there is a problem with hidden stations. This isa problem inherent in the CSMA system and to alleviate it the systemdefines a request-to-send/clear-to-send (RTS/CTS) mechanism that can beused as an option. Before data frames are transmitted, it is possiblefor a system to send a short RTS frame, which is followed by a CTStransmission from the receiving station. The RTS and CTS frames containinformation on the length of the transmission time of the next dataframe, i.e. the first fragment, and of the corresponding ACK response.What is achieved in this way is that other stations near thetransmitting station, and hidden stations near the receiving station, donot start a transmission, because they set a counter, the what is calledNetwork Allocation Vector (NAV). The RTS/CTS mechanism helps to protectlong data frames against hidden stations. With fragmentation, a largenumber of ACKs are transmitted, whereas with RTS/CTS the MSDU can betransmitted efficiently in a single data frame. Between each successivepair of frames in the sequence RTS frame, CTS frame, data frame and ACKframe, there is a short interframe space (SIFS), which is 16 μs longunder 802.11a.

FIG. 1, which relates to the prior art, is a diagram showing an exampleof a distributed coordination function (DCF). A short interframe space(SIFS) is shorter than a DCF interframe space (DIFS), as a result ofwhich CTS responses and acknowledgement frames (ACKs) always have thehighest priority for access to the wireless medium. In the latestversion of the MAC protocol, the 802.11e protocol, an EnhancedDistribution Coordination Function (EDCF) has been introduced, whichstill operates in the same way but, in addition, supports differenttypes of traffic, such as, for example, access priorities. In thetime-based diagram for six stations shown in FIG. 1, although station 6cannot detect the RTS frame of the station 2 that is transmitting, itcan detect the CTS frame of station 1.

Another known function, the Hybrid Coordination Function (HCF) extendsthe rules for (E)DCF access. A crucial performance feature of 802.11eMAC is the Transmission Opportunity (TXOP). A TXOP is defined as theinterval between the point at which a station receives the right toinitiate a transmission, defined by the starting time, and a maximumduration. TXOPs are allocated by way of contentions (EDCF-TXOP) or aregranted by HCF (polled TXOP). Only one station in the cell, which iscalled the hybrid coordinator (HC), can give other stations permissionto transmit, i.e. can grant a TXOP. The duration of a polled TXOP isspecified by the time field within the allocating frame. The hybridcoordinator is able to allocate TXOPs to itself, to enable MSDUtransmissions to be initiated, at any time, but only on detecting thatthe channel is idle for a time equal to a PIFS (Point CoordinatorInterframe Space), which time is shorter than the length of a DIFS.

Defined as part of the 802.11e protocol is an additional random accessprotocol that enables collisions to be reduced. What is calledControlled Contention gives the hybrid coordinator an opportunity oflearning what stations need to be queried at what times with regard totheir wishes to transmit. The controlled contention mechanism allowsstations to request to be allocated polled TXOPs by sending a sourcequery, without interfering with other (E)DCF traffic.

It is an object of the invention to specify methods for medium access bya multi-channel device on a transmission system having at least twochannels, which methods give terminals not having the performancefeature of channel grouping the opportunity of transmitting andreceiving on said transmission systems, i.e. methods that make itpossible for single-channel and multi-channel devices to coexist.Methods are to be specified for a centralized mechanism and adecentralized mechanism.

The object is achieved in accordance with the invention by a method foraccess to a medium by a multi-channel device, which medium comprises atransmission system having at least two channels, on which a message tobe transmitted comprises at least a preamble, a header and a succeedingcontrol or data section of a frame, and the preamble and header of themessage are repeated on all the channels. As a result of the controlinformation in a frame being repeated on every channel, even asingle-channel device is able to pick up the preambles and headers andperform a standards-compliant waiting process, possibly with a randombackoff time. If the PHY and the MAC headers were not repeated on everyfrequency channel, single-channel devices would not recognize thetransmission and would interpret it is as interference that blocks theirchannel for an indeterminate length of time.

In one embodiment of the invention, the preamble and header are repeatedin parallel on all the channels. The parallel transmission of theseparts of the message cannot begin until all the channels are free.Simulations have shown that the loss in data transmission rate thatoccurs as a result of the parallel transmission is relatively slight.

In a further embodiment, the succeeding control or data section of aframe is taken from the group request-to-send (RTS), clear-to-send(CTS), acknowledgement (ACK) or data (DATA).

In a special embodiment of the invention, the multi-channel deviceoperates to standard IEEE 802.11, i.e. 802.11e or 802.11n having amedium access control (MAC) protocol, and it is not only the preamblesand headers that are repeated on all the channels but also at least someof the items of information belonging to the MAC protocol.

If the medium access takes place under standard IEEE 802.11, i.e.802.11e, the RTS, CTS and ACK control frames are transmitted on all thechannels, which means that the single-channel devices set their networkallocation vectors (NAVs) on the basis of the information in the RTS/CTSdata packets. The setting of the NAV causes a counter to start, and noattempt is made to access the radio transmission medium until a targetcount has been reached.

The object is also achieved in accordance with the invention by a methodfor access to a medium by a multi-channel device, which medium comprisesa transmission system having at least two channels that themulti-channel device intends to call upon for transmission, which methodhas the following steps

-   -   scanning by the multi-channel device of all the channels to be        called upon for transmission,    -   finding that a single one of these channels is idle or that a        back-off by the device itself is underway on this channel,    -   blocking of this channel to other devices by the multi-channel        device,    -   further scanning of the other channels to be called upon and        blocking or reserving thereof on finding that the channel        concerned is idle or that a backoff is underway thereon.

What is finally achieved as a result of the successive blocking ofindividual channels is a state where all the channels to be called uponfor transmission are idle. Other devices that scan a blocked channelrecognize that the channel is not idle and therefore do not themselvesbegin a transmission. The transmission of the message can then beperformed with grouped channels and hence at a high data rate.

In one embodiment, the blocking of a channel that is idle is performedby the multi-channel device and the receiving device, each of whichemits a reserving message.

In one embodiment, the reserving message is implemented in the form ofRTS and CTS frames that are transmitted by the following steps

-   -   transmission of an RTS frame on the free channel by the        multi-channel device, so that devices in the area surrounding        the multi-channel device that is transmitting will set their        NAVs,    -   transmission of a CTS frame on the free channel by the receiving        device, so that stations in the area surrounding the receiving        station will set their NAVs.

The multi-channel device is able to carry out its transmission withchannel grouping in this case, on all the channels that it has itselfpreviously blocked.

In one embodiment, the blocking of a channel that is idle for apredetermined period of time is performed by starting the transmissionby the multi-channel device on the single channel, doing soalternatively with or without an RTS-CTS mechanism. What the RTS-CTSmechanism entails is

-   -   optional transmission of an RTS frame on the free channel by the        multi-channel station,    -   optional transmission of a CTS frame by the receiving station        and    -   transmission of a data frame on the free channel by the        multi-channel station (and hence blocking of the channel).

The object is also achieved in accordance with the invention by a methodfor access to a medium by a multi-channel device, which medium comprisesa transmission system having at least two channels that themulti-channel device intends to call upon for transmission, in whichmethod a third device (independent of the transmitter and receiver)reserves or blocks the channels in the channel group for themulti-channel device that wishes to transmit. This third device hasassumed responsibility on the network for emitting the synchronizingbeacon.

In one embodiment, the third device is responsible for coordinatingmedium access to a plurality of channels.

In one embodiment, in the event of individual channels in the channelgroup not becoming free simultaneously, the third device causes,alternatively

-   -   (a) one channel or individual channels to be blocked until such        time as all the channels in the channel group have become free,        or    -   (b) a channel that has become free to be assigned immediately to        the multi-channel device that wishes to transmit.

In one embodiment, the medium access is performed under standard IEEE802.11, i.e. 802.11e or 802.11n, and said third device is generally theso-called hybrid coordinator or point coordinator. The centralizedreservation of the channels is performed by the third device, which isresponsible for emitting the beacon on all the channels in the channelgroup and at the same time keeps free or reserves all the channels inthe channel group for the multi-channel station that wishes to transmit.If said third device or station is responsible for coordinating mediumaccess to all the channels, it can make the reservations for the deviceson all the channels in such a way that phases able to be usedsimultaneously are provided for the multi-channel devices on all thechannels in their channel groups. Should the individual channels in thechannel group nevertheless not become free simultaneously, there are twodifferent ways in which said third station is able to secure the channelfor the multi-channel station until all the channels in the channelgroup become free.

In a preferred embodiment of the method, the medium access is performedunder standard IEEE 802.11, i.e. 802.11e or 802.11n, and said thirddevice is the hybrid coordinator or point coordinator.

In a further embodiment of the method, the point coordinator or hybridcoordinator transmits what is called beacons in parallel on all thechannels. The transmission may if required be performed simultaneouslyor in synchronized fashion.

The method in accordance with the invention can be employed on atransmission system using the Standard Universal MobileTelecommunication System (UMTS).

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 2 is a time-based diagram of a prior art transmission.

FIG. 3 is a time-based diagram of a first variant of the method inaccordance with the invention.

FIG. 4 is a time-based diagram of a second variant of the method inaccordance with the invention.

FIG. 2 is a time-based diagram of a prior art transmission. Thetransmission always takes place on only one channel even when a channelis idle, as channel 3 (ch 3) is in the present case.

FIG. 3 is a time-based diagram of a first variant of the method inaccordance with the invention. Possible examples of messages are shownon individual ones of six channels. Two stations usually communicate ona channel by alternately transmitting an RTS frame, a CTS frame, a DATAframe and an ACK frame. At time T₁, a multi-channel device intends tostart transmitting data using channel grouping. In the present example,scanning of the six channels shows that channels 1, 2 and 5 are idle,whereupon, following a preamble and header PR, an RTS frame is firsttransmitted and a CTS frame is then received. On reception of the threeCTS frames, which are parallel in this case, channels 1, 2 and 5 areblocked, which means that any other stations, regardless of whether theyare single-channel or multi-channel devices, have set their NAVs. Attime T₂, the multi-channel device detects that channel 4 is idle,whereupon the RTS-CTS procedure is performed and this channel is thusblocked. The same thing happens with channel 3 at time T₃. Once five outof six channels have been blocked, the transmission of data starts attime T₄, when the final channel too, namely channel 6, is recognized tobe idle. The preamble and header PR having been transmitted on the sixindividual channels, the transmission of the data DATA takes place usingchannel grouping, by which means a higher data rate is attained. Oncompletion of the transmission of the data, the transmittingmulti-channel device receives on each individual channel, following apreamble and header PR, an acknowledgement frame ACK. An advantage ofthis variant is that the transmission of data starts at a fixed, highdata rate right at the outset and does so simultaneously on all thechannels.

FIG. 4 is a time-based diagram of a second variant of the method inaccordance with the invention. In this example, the messages and thebusy/idle states on the channels before the scanning by themulti-channel device begins are assumed to be the same as in FIG. 3.However, on channels that are in the idle state, the transmission ofdata is begun for the purpose of blocking them, which happens at time T₄in this case. On other channels, transmission begins (possibly after apause) as soon as it is detected that a channel is idle, which isdetected to be the case on channel 4 at time T₅ and on channel 3 at timeT₆ in the present case. At time T₇, the final channel, namely channel 6,is brought in as well. On this channel, the message first begins with apreamble and header. Following this, channel grouping then takes placefor the DATA frame using all six channels. At the beginning T₄ of thetransmission of the data, three individual channels are used in paralleland only at time T₇ is the bandwidth increased by channel grouping.

It is true of all three diagrams that a space is shown between the endof a control frame and the start of a preamble and header, which spaceis intended to show that there may be a brief pause here.

The invention claimed is:
 1. A method for accessing a medium by amulti-channel device, in which the medium comprises a transmissionsystem having at least two channels, the method comprising: recognizingan idle state and a back-off state; determining whether the idle stateor the back-off state is underway on each channel of the at least twochannels that are an object of channel grouping, transmitting a messageincluding a preamble and header (PR) and a control section on eachchannel determined to be either idle or having the back-off underway ofthe at least two channels that are an object of channel grouping toreserve the at least two channels, such that a single channel devicedetects the preamble and header and performs a process according tocontrol information included in the control section, wherein thepreamble and header (PR) are repeated in parallel on the at least twochannels.
 2. The method of claim 1, wherein the message is one of arequest-to-send (RTS), clear-to-send (CTS), or acknowledgement (ACK)type.
 3. The method of claim 1, wherein the multi-channel deviceoperates in compliance with IEEE 802.11 standard and a medium accesscontrol (MAC) protocol, the method further comprises repeatinginformation belonging to the MAC protocol on the at least two channels.4. The method of claim 1, wherein access to the medium takes place underIEEE 802.11 standard, the method further comprising transmittingrequest-to-send (RTS), clear-to-send (CTS), and acknowledgment (ACK)control frames on the at least two channels, and setting networkallocation vectors (NAVs), by single channel devices, based oninformation in the RTS/CTS control frames.
 5. The method of claim 1,further comprising: employing Standard Universal MobileTelecommunication System (UMTS) as the transmission system.
 6. A methodfor accessing a medium by a multi-channel device, the medium including atransmission system having at least two channels that the multi-channeldevice intends to call upon for transmission, the method comprising:scanning, by the multi-channel device, the at least two channels to becalled upon for transmission, recognizing an idle state and a back-offstate; determining whether the idle state or the back-off state isunderway on a single one of the scanned channels; blocking the singlechannel determined to be one of either idle or having the back-offunderway to other devices by the multi-channel device by transmitting amessage including a preamble and header (PR) and a control section,wherein the preamble and header (PR) are repeated in parallel on the atleast two channels, such that a single channel device detects thepreamble and header and performs a process according to controlinformation included in the control section, further scanning the otherchannels to be called upon for transmission and blocking or reservingthe other channels based on a determination that a scanned channel isone of either idle or that a back-off is underway by transmittinganother message on the scanned channel.
 7. The method of claim 6,further comprising: blocking the channel by the multi-channel device anda receiving device, each of the devices emitting the message.
 8. Themethod of claim 7, wherein the message is implemented in the form of RTSand CTS frames, the method further comprising: transmitting an RTS frameon a free channel by the multi-channel device, so that devices in thearea surrounding the multi-channel device that is transmitting will settheir network allocation vectors (NAVs), and transmitting a CTS frame onthe free channel by the receiving device, so that stations in the areasurrounding the receiving station will set their NAVs.
 9. The method ofclaim 7, further comprising transmitting with channel grouping, by themulti-channel device, on all channels that it has previously blocked.10. The method of claim 6, further comprising blocking a channel bystarting the transmission by the multi-channel station on the singlechannel, wherein the transmission can be made with or without an RTS-CTSmechanism.
 11. A method for accessing a medium by a multi-channeldevice, the medium comprises a transmission system having at least twochannels that the multi-channel device intends to call upon fortransmission, wherein a message to be transmitted on the mediumcomprises a preamble and a header (PR) followed by at least one of acontrol section or data section, the method comprising: scanning the atleast two channels to be called upon for transmission, recognizing anidle state and a back-off state; determining whether the idle state orthe back-off state is underway on each channel of the at least twochannels to be called upon for transmission, repeating the preamble andheader (PR) of the message on all channels to be called upon fortransmission that are determined to be either idle or having a back-offunderway, and reserving or blocking, by a third device independent of atransmitter and receiver of the message, the channels in a channel groupfor the multi-channel device that intends to transmit, such that asingle channel device detects the preamble and header and performs awaiting process.
 12. The method of claim 11, further comprising:coordinating, by the third device, access to the medium for a pluralityof channels.
 13. The method of claim 11, wherein in the event ofindividual channels in the channel group not becoming freesimultaneously, the third device causes, alternatively, blocking onechannel or individual channels until such time as all the channels inthe channel group have become free, or assigning a channel that hasbecome free immediately to the multi-channel device that intends totransmit.
 14. The method of claim 11, wherein the third device is ahybrid coordinator or point coordinator, the method performing themedium access under standard IEEE 802.11.
 15. The method of claim 14,further comprising: transmitting, by the point coordinator or hybridcoordinator, beacons in parallel on all the channels.
 16. Amulti-channel device for accessing a medium, the medium comprises atransmission system having at least two channels, the multi-channeldevice performing the method of claim 1 for accessing the medium.
 17. Awireless network comprising a transmission system having at least twochannels and at least one multi-channel device as claimed in claim 16.18. A communication device comprising: one or more transmit antennasconfigured to transmit on at least two channels; and a computerprocessor coupled to the one or more transmit antennas and configured torecognize an idle state and a back-off state, determine whether the idlestate or the back-off state is underway on each channel of the at leasttwo channels that are an object of channel grouping, transmit a messageincluding a preamble and header (PR) and a control section on eachchannel determined to be either idle or having the back-off underway ofthe at least two channels that are an object of channel grouping toreserve the at least two channels, such that a single channel devicedetects the preamble and header and performs a process according tocontrol information included in the control section, wherein thepreamble and header (PR) are repeated in parallel on the at least twochannels.
 19. The communication device of claim 18, wherein the messageis one of a request-to-send (RTS), clear-to-send (CTS), oracknowledgement (ACK) type.
 20. The communication device of claim 18,wherein the multi-channel device operates in compliance with IEEE 802.11standard and a medium access control (MAC) protocol, the method furthercomprises repeating information belonging to the MAC protocol on the atleast two channels.
 21. The communication device of claim 18, whereinaccess to the medium takes place under IEEE 802.11 standard, thecomputer processor being further configured to transmit or receiverequest-to-send (RTS), clear-to-send (CTS) and acknowledgment (ACK)control frames on the at least two channels, and wherein networkallocation vectors (NAVs) are set, by single channel devices, based oninformation in the RTS and CTS control frames.
 22. A communicationdevice comprising; one or more transmit antennas configured tocommunicate utilizing at least two channels of a transmission system;and a computer processor coupled to the one or more transmit antennasand configured to scan the at least two channels, recognize an idlestate and a back-off state, determine whether the idle state or theback-off state is underway on a single one of the scanned channels,block the single channel determined to be one of either idle or havingthe back-off underway to other devices by the multi-channel device bytransmitting a message including a preamble and header (PR) and acontrol section, wherein the preamble and header (PR) are repeated inparallel on the at least two channels, such that a single channel devicedetects the preamble and header and performs a process according tocontrol information included in the control section, the computerprocessor being further configured to scan the other channels and blockor reserve the other channels based on a determination that the scannedchannel is one of either idle or that a back-off is underway bytransmitting another message on the scanned channel.
 23. Thecommunication device of claim 22, wherein the message is implemented inthe form of transmit request-to-send (RTS) and clear-to-send (CTS)frames, the computer processor being configured to transmit an RTS frameon a free channel by the multi-channel device, and receive a CTS frameon the free channel from the receiving device.
 24. The communicationdevice of claim 22, wherein the computer processor is configured totransmit on all channels that were previously blocked.
 25. Thecommunication device of claim 22, wherein the computer processor isconfigured to block a channel by starting the transmission by themulti-channel station on the single channel, wherein the transmissioncan be made with or without an RTS-CTS mechanism.
 26. A communicationdevice comprising; one or more transmit antennas configured tocommunicate utilizing at least two channels of a transmission system;and a computer processor coupled to the one or more transmit antennasand configured to scan the at least two channels to be called upon fortransmission, recognize an idle state and a back-off state, determinewhether the idle state or the back-off state is underway on each channelof the at least two channels to be called upon for transmission, andrepeat the preamble and header (PR) of the message on all channels to becalled upon for transmission that are determined to be either idle orhaving a back-off underway, wherein a message to be transmitted on themedium comprises a preamble and a header (PR) followed by at least oneof a control section or data section, and wherein channels in a channelgroup of the communication device are reserved or blocked by a thirddevice independent of a transmitter and receiver of the message suchthat a single channel device can detect the preamble and header andperform a waiting process.
 27. The communication device of claim 26wherein access to the medium for a plurality of channels is coordinatedby the third device.